It's a joke. Even someone as well known as Peter Norvig is unlikely to be so gauche as to name a "law" after himself except tongue in cheek.

It seems to be intended just as a common-sense reminder that fast growth has to eventually slow/stop due to market saturation.

It's not strictly true though since the market itself can grow so your sales could still double or more from a level that had represented 50% of the market at some time in the past.

Everything true is tautological in some context.